The subject matter disclosed herein generally relates to directional control of aircraft. More specifically, the subject disclosure relates to yaw control of dual coaxial rotor rotary wing aircraft.
A rotary wing aircraft, or helicopter, with a coaxial contra-rotating rotor system is capable of higher speeds as compared to conventional single rotor helicopters due in part to the balance of lift between advancing sides of the main rotor blades on the upper and lower rotor systems. Such aircraft, however, tend to have weak yaw control during hover flight, low speed flight conditions, under low rotor thrust conditions and during autorotational landings. Low speed/low rotor thrust flight conditions are encountered in aircraft return to target and similar “low g” maneuvers. The ability to safely autorotate is an important feature, especially for single engine aircraft. The poor yaw performance is typically due to the inability of the aircraft to create adequate differential rotor torque between the two coaxial rotors under these flight conditions.